This invention pertains to ignition circuits, and, more specifically, to an ignition circuit incorporating a piezoelectric transformer that improves the efficiency of a spark ignition system.
Spark ignition systems are designed to generate high voltages and deliver energy at sufficient quantity and speed to ensure arc breakdown across a gap. Breakdown voltages depend on the application, e.g., internal combustion, gas turbine engines, energetic material initiation, etc. These voltages must be generated with a reasonable degree of certainty to attain accurate timing for the ensuing combustion process. The voltage and resulting energy must also be generated with a suitable margin to account for conditional changes from event to event that change the absolute breakdown voltage.
Spark ignition systems also deliver energy during and post breakdown to sustain combustion. Post-breakdown, the ignition system should provide enough energy to the ionized plasma to sustain the kinetics of local ignition. For many ignition systems, it is not sufficient to merely provide the energy (see Automotive Handbook, 2nd edition, Robert Bosch GmbH, 1986). The quantity and duration of energy delivery, including energy delivery as a function of time, also influences combustion efficiency. Delivering post-breakdown energy too quickly, and thus at too high a current, results in electrode wear and unsustainable combustion, while delivering the energy too late simply heats the post-combustion products.
Many examples of ignition systems requiring control over both absolute timing and energy delivery profiles are found in internal combustion engines (Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw-Hill, 1988; Bosch Automotive Handbook; G. F. W Ziegler, et al., xe2x80x9cInfluence of a Breakdown Ignition System on Performance and Emission Characteristics,xe2x80x9d SAE Technical Paper Series, No. 840992, 1984; C. F. Edwards and A. K. Oppenheim, xe2x80x9cA comparative study of plasma ignition systems,xe2x80x9d SAE Technical Paper Series, No. 830479, 1983; Nakai, M., et al., xe2x80x9cStabilized Combustion in a Spark Ignited Engine through a Long Spark Duration,xe2x80x9d SAE Technical Paper Series, No. 850075, 1985; U.S. Pat. Nos. 3,838,671, 5,024,204, and 5,383,433). Typically, these systems must initiate combustion during startup conditions and run cleanly and efficiently through changes in a variety of conditions such as mixture ratios, intake pressures, and cylinder temperatures.
Small internal combustion (IC) engines, such as those found in lawn tractors and mowers, snow blowers, marine in/outboard motors, etc., pose unique requirements for engine startup and ensuing transition to operation speeds. Hand crank speeds and resulting cylinder pressures are low, requiring startup spark timing very close to top dead center (TDC) and spark durations on the order of several milliseconds. After start, the engine speed continues to increase, requiring increased spark advance and reduced spark energies for efficient running at operation speeds. It may not be cost efficient to accommodate both startup and steady operating conditions within the same ignition system. Further, larger IC engines depend on weighty and expensive xe2x80x9cfirst timexe2x80x9d starter systems. Typically, these involve electronic fuel pumps, starter motors and electronic ignition systems all working in unison to provide first time starts in the most adverse of conditions. Due to the conditions at start-up, e.g., cold oils, cold cylinders, cold catalytic converters, etc., excessive amounts of fuel are typically injected into the combustion chamber, only a fraction of which burns, resulting in high emissions. Recently, more attention has been paid to these start-up emissions because emissions during steady engine operations are decreasing.
Full electronic control of automotive ignition systems has enabled multidimensional timing maps as a function of speed, load, intake temperature, engine temperature and various other sensed variables (see, e.g., Bosch Automotive Handbook). These timing maps represent complex, empirically determined engine data for absolute timing of ignition systems. It is an object of the present invention to provide both timing and energy delivery control in electronic ignition. This extra degree of freedom for controlling engine performance (of all size engines) will reduce emissions and improve engine performance goals.
General aviation ignition systems require extremely reliable breakdown and energy delivery in order to guarantee ignition. Typical aircraft IC ignition systems, such as those sold commercially by Unison Industries, deliver a peak voltage of 23 kiloVolts (un-fouled, maximum required) and an energy discharge of 50 milliJoules. Some of these IC systems provide multiple sparks per sequence in order to ensure ignition.
In contrast, gas turbine ignition systems require adjustment of the firing rate for changing environmental conditions, while their absolute firing times are not as critical as in an IC engine. U.S. Pat. No. 5,852,381, issued Dec. 22, 1998, describes an exciter for generating voltages and energies necessary for turbine ignition. Typically, these systems fire once to twice a second and require high spark energy, on the order of Joules. Commercial exciter systems, such as that disclosed in the ""381 patent, are controlled by regulatory electronics that are independent of the passive electrical properties of the igniter plug but are disposed in the high voltage path, making them susceptible to failure. Furthermore, current turbine engine exciter systems are based on captive discharge and thus generate high current rates that ultimately result in wear and electronic part fatigue.
In the defense industry, typical mono- and bi-propellant weapons fuels, such as hydrazine, provide unique challenges. They are generally highly toxic and have high transport, handling, storage, and other logistical costs. Suggested replacement fuels, such as hydrogen peroxide oxidants, generally have higher heat capacity and are harder to ignite. Enabling simple, lightweight ignition of these fuels could result in huge defense cost savings.
It is a further object of the present invention to provide a controllable system for non-ignition breakdown applications. Such applications include pest killers, electrostatic discharge weapons (Taser), safe and arm devices, and pyrotechnic initiators and actuators. In particular, through optimal power delivery, the present invention is intended to enable low cost, low power, rapid response pyrotechnic actuators such as air bag detonators.
The invention is a tuned power ignition (TPDI) system employing a piezoelectric transformer. The invention includes both means for tuning output electrical circuit impedance and specific timing control of the transformer. Together, these elements generate breakdown voltage and post-breakdown energy with deliberate quantity and accuracy. Parameters of the TPDI system are optimized with respect to the particular combustion system, e.g., an IC engine system, etc.
The timing control may be a function of a predetermination or calibration of system performance. Exemplary input signals for the timing control also include tuned output circuit impedance, predetermined external system calibration, and parameters measured by external system sensors. With tuned electrical output impedance, the power flow across the spark gap is optimized such that, when combined with accurate timing inputs, controlled timing of breakdown and duration of energy delivery is made possible. Through precise timing inputs, TPDI output power is converted to regulated energy delivery.
Typical ignition systems, for example, magnetically transformed and captive discharge, provide control over timing of breakdown voltage and limited quantities of post-breakdown energy. In contrast, TPDI provides a tool for controlling both absolute timing of breakdown and relative timing and quantity of the post-breakdown energy.
TPDI can be also used as the sole ignition system for IC engines of all sizes. It can also be used as a starter system for IC engines, either as a simple parallel add-on or a means for reducing the size, weight and cost of the starter motor. TPDI has utility as an initiator for energetic materials commonly used in detonators and pyrotechnic actuators. The system may also be exploited in starters and ignition systems for general and commercial aviation industries. In addition, the system may be incorporated into pest killers, electrostatic discharge weapons (Taser), and safe and arm devices.
In one aspect, the invention is an ignition system. The ignition system includes a piezoelectric transformer having a drive side, an output side, and a piezoelectric element circuit elements in electronic communication with the output side that tune output impedance in series with a breakdown gap to optimize power flow from the transformer to the breakdown gap after breakdown, and a timing control circuit in electronic communication with the drive side that meters post-breakdown energy delivered to the breakdown gap by timing the duration of post-breakdown power flow. The system additionally includes electronic feedback control, feed forward control, or both. These optimize output performance as the resonance of the transformer changes. The feedback control may receive a signal from the piezoelectric element, respond to an impedance in series electronic communication with the output side of the transformer, or both. The piezoelectric transformer may be a multi-layer Rosen type transformer, or a multi-layer thickness-extensional mode type transformer. The circuit elements may include a resistor, an inductor, a capacitor, or any combination of these.
The ignition system may further include sensor inputs that provide data for the timing circuit to determine the absolute timing and length of a timing pulse. The timing pulse and the tuned impedance determine the net energy and power delivery to the breakdown gap post-breakdown. The sensor inputs may measure one or more of output voltage, output power delivered, revolutions per minute, torque load, throttle position, pressure intake temperature, exhaust temperature or composition, intake composition, fuel consumption, humidity, catalytic converter or cylinder wall temperature, a temperature distribution across a turbine blade, spark discharge current, combustion performance, drive side voltage, and external control computer outputs. The sensor inputs may also measure chemical species, for example, oxygen, carbon monoxide, and carbon dioxide.
The ignition system may be used in addition to a pre-existing ignition system to optimize breakdown timing and post-breakdown energy delivery during pre-determined operating conditions. For example, the ignition system may be used during start up. Alternatively, the pre-existing ignition system may break down gap, and the ignition system may regulate the post-breakdown energy discharge.
The timing control circuit may provide an output signal to one or more of a transformer drive side amplifier, an oscillation generator in electronic communication with the drive side, and the feedback control. The timing control and the feedback and/or feed forward control may be combined in an integrated circuit. The output signal may be generated in response to parameters measured during operation of the ignition system. The timing control circuit may compare the parameters to a pre-determined reference value. The output signal may be a function of a plurality of measurements of the parameters. The output signal may be generated independently of the parameters. In addition, the timing control circuit may generate an output signal as a general waveform pulse having multiple control levels, a series of relatively timed single control level pulses, or both.
The transformer drive electronics may operate around a bias voltage to lower cycle by cycle resonant frequency variations and minimize material hysteresis. The piezoelectric transformer may include a single crystal piezoelectric element. The ignition system may include a plurality of piezoelectric transformers each having output rectification diodes that are adapted and constructed to be in electrical communication with a single capacitor that provides a charge to the breakdown gap.
In another aspect, the invention is a device having a combustion engine that has an igniter. The igniter has a primary ignition system and a secondary ignition system. The primary ignition system includes a power source and a capacitor that is charged by a power source. The secondary ignition system includes a piezoelectric transformer, circuit elements that tune the transformer output impedance in series with a breakdown gap, and a timing control circuit that meters post-breakdown energy delivered to the breakdown gap. The secondary system also includes an electronic feedback control, feed forward control, or both.
In another aspect, the invention is an ignition system including a piezoelectric transformer having a drive side, an output side, and a piezoelectric element, means for tuning an output impedance of the transformer in series with a breakdown gap, and means for metering post-breakdown energy delivered to the breakdown gap. The means for tuning optimize power flow from the transformer to the breakdown gap after breakdown. They may include a resistor, an inductor, a capacitor, or any combination of these in series electronic communication with the breakdown gap. The system further includes means for optimizing output performance of the transformer as a resonance condition of the transformer changes. The means for optimizing may include electronic feedback control, feed forward control, or both. These controls respond to an impedance in series electronic communication with the output side of the transformer.